Summary:
The Ord instance for ModuleName is currently implemented in
terms of Uniques causing potential determinism problems.
I plan to change it to use the actual FastStrings and in
preparation for that I'm switching to UniqFM where it's
possible (you need *one* Unique per key, and you can't get
the keys back), so that the performance doesn't suffer.

Test Plan: ./validate

Reviewers: simonmar, austin, ezyang, bgamari

Reviewed By: bgamari

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2320

GHC Trac Issues: #4012